This invention relates to an objective lens driving apparatus used in an optical pickup for an optical disk drive.
An optical disk drive is an apparatus for reading information recorded in an optical disk (such as CD, CD-ROM, CD-R, CD-RW, DVD-ROM, DVD+R, DVD−R, DVD-RAM, DVD+RW, and DVD−RW) and writing information into the optical disk. The optical disk drive of the type has an optical pickup for irradiating a laser beam onto the optical disk and detecting a reflected beam from the optical disk in order to read information from the optical disk and write information into the optical disk.
Generally, the optical pickup comprises a laser source for emitting the laser beam and an optical system for guiding the laser beam to the optical disk and guiding the reflected beam to an optical detector. The optical system contains an objective lens faced to the optical disk.
The objective lens used in the optical pickup must be accurately controlled in position with respect to a focusing direction along an optical axis and a tracking direction along a radial direction of the optical disk so as to precisely converge or focus the laser beam onto a recording surface (track) of the optical disk which is rotated. Recently, following the improvement in recording density, there is an increasing demand for removal or suppression of the influence of warping of the optical disk. Therefore, the objective lens must be controlled in tilt also.
The objective lens is held and driven by an objective lens driving apparatus. The objective lens driving apparatus comprises a lens holder holding the objective lens and a plurality of suspension wires elastically supporting the lens holder so as to enable focusing control, tracking control, and tilting control. The lens holder is provided with a focusing coil, a tracking coil, and a tilting coil each of which is wound on or attached to the lens holder. Each of these coils is partly located within a gap of a magnetic circuit. With the above-mentioned structure, the objective lens driving apparatus can finely control the position and the tilt of the objective lens by controlling an electric current flowing through each of the coils (for example, see Japanese Patent Application Publication (JP-A) 2001-93177).
Hereinafter, a conventional objective lens driving apparatus disclosed in the above-mentioned publication will be described with reference to the drawing.
Referring to FIG. 1, the conventional objective lens driving apparatus comprises an objective lens 11, a lens holder 12, a focusing coil 13, a plurality of tracking coils 14, a plurality of tilting coils 15, a pair of yokes 16, a pair of magnets 17, a plurality of suspension wires 18, and a base 19.
The objective lens 11 is disposed on an upper surface of the lens holder 12. The focusing coil 13 is wound around side surfaces of the lens holder 12. The tracking coils 14 and the tilting coils 15 are attached to the side surfaces of the lens holder 12.
Each of the suspension wires 18, four in number (two of them being illustrated in the figure), has one end fixed to the lens holder 12 and the other end fixed to the base 19. With this structure, the lens holder 12 is elastically supported to be movable in the focusing direction (up-and-down direction in the figure) and in the tracking direction (forward-and-rearward direction in the figure) and to be tiltable in the radial direction of the optical disk (circumferential direction of a rotation axis extending in the leftward-and-rightward direction in the figure).
A combination of one of the yokes 16 and one of the magnets 17 forms a magnetic circuit. Likewise, a combination of the other yoke 16 and the other magnet 17 forms another magnetic circuit. In a magnetic gap in each of the magnetic circuits, each of the focusing coil 13, the tracking coils 14, and the tilting coils 15 is partly located.
In the above-mentioned structure, when each of the focusing coil 13, the tracking coils 14, and the tilting coils 15 is supplied with an appropriate electric current, the lens holder 12 (and the objective lens 11) can be moved and/or tilted in response to the electric current. As a consequence, the laser beam can be properly focused on a track on a recording surface of the optical disk.
As described above, the lens holder is supported by the four suspension wires in a cantilevered fashion. Therefore, when focusing shift by the focusing control and tracking shift by the tracking control simultaneously occur, a force twisting the suspension wires (in the radial direction) is produced. This force makes the lens holder (i.e., the objective lens) be tilted with respect to the optical disk.
The tilting coil may be used in order to correct such tilting. However, if the magnitude of tilting (tilt change) is large, correction is impossible. Therefore, in order to prevent the shift of the lens holder from being significantly biased in a specific direction, the magnets are disposed so that their centers in a height direction are coincident with the center of driving (neutral position) of the lens holder.
In the meanwhile, a two-wavelength optical pickup is known which is adaptable for both of a CD (Compact Disk) and a DVD (Digital Video or Versatile Disk). In the two-wavelength optical pickup, the position of the objective lens in the focusing direction must be changed between the CD and the DVD as the optical disk. In other words, an objective lens driving apparatus used in the two-wavelength optical pickup must have two reference positions (reference of driving) for the CD and the DVD. It is assumed here that one of these reference positions is coincident with the center of driving of the lens holder. In this event, the other reference position is widely separated from the center of driving and the displacement caused by the focusing control and the tracking control will be large. Thus, if a position separated from the center of driving is used as the reference position, a large force exceeding an allowable range may possibly be produced in the radial direction if the shift of the lens holder is great. In view of the above, in the objective lens driving apparatus used in the two-wavelength optical pickup, the center of driving of the lens holder is located between the reference positions for the CD and the DVD. More specifically, the reference position for the CD is nearer to the optical disk than the center of driving while the reference position for the DVD is further from the optical disk than the center of driving.
However, due to production variation, the objective lens driving apparatus used in the two-wavelength optical pickup may sometimes exhibit a tilt change beyond an allowable range if the lens holder is shifted by a predetermined amount. Thus, the objective lens driving apparatus is disadvantageous in that a margin for the tilt change is small.